1. Field of the Invention
The present innovations relate to magnetic recording. More particularly, an innovative adaptive equalization system and method is disclosed.
2. Background of the Invention
Storage technology, particularly magnetic storage, is driven by a need to increase storage density and reduce the size of storage hardware. As storage density increases and size decreases, the tolerances for reading and writing data become more strict. Data density is ultimately driven by limitations of minimizing the length and width of the transition signal at the media.
When certain data patterns occur during high-density data recording on magnetic media, non-linear bit shifts arise. These bit shifts are primarily caused by demagnetization effects, i.e., as a result of the fact that the magnetic medium ‘opposes’ the recording of a change of sign in the bit stream, particularly when a plurality of bits of the same sign precede one bit of an opposite sign. This effect manifests itself in that a change of sign or transition is recorded too late or too early, which results in bits being recorded which are too short or too long, respectively.
The article “Write current equalization for high speed digital magnetic recording” by T. Kato, R. Arai and S. Takanami in IEEE Trans. Magn., Vol. MAG-22, No. 5, pp. 1212-1214, September 1986, describes the problem of non-linear bit shifts as a result of the limited bandwidth of the recording channel, particularly as a result of the self-inductance of the write head.
Write equalization adds short duration pulses to the write current, each of which consists of two transitions. The added pulses have too short a wavelength to be resolved during readback. Their effect is to slim readback pulses resulting from data 1 transitions. This enhances the performance of certain peak detect and PRML channels.
As size decreases, new sources of magnetic spacing loss arise. Pole tip recession (PTR) is one such source. As flying heights reach near-contact levels, variations in PTR may represent a significant fraction of the total distance between the read/write head and the magnetic media.
Magnetic recording systems that use write equalization experience significant variation in the overall equalization due to the effects of PRT. The high-density transitions written in order to accomplish the write equalizer transfer function become the shortest wavelengths of the system. Hence, they are the most sensitive to variations in separation between the head and the tape. PTR differs from the normal variation in separation because it changes slowly over the life of the product, or is at least constant for each region of a tape written by a specific system. The normal head to media separation variation experienced while recording needs to be accounted for with a method such as parallel decoders with separate equalization or adaptation such as LMS (Least Mean Squared). However, parallel decoders with separate equalization in its optimal configuration may not span the necessary frequency variation, and implementing LMS can be costly. LMS options also are less desirable because of their delayed response time in a decoding scheme.
The current technology of magnetic recording would therefore benefit from an effective means to overcome PTR or other systematic variations.